There have been dramatic improvements in the design and performance characteristics of compact hermetically sealed rechargeable electrochemical cells. These cells are typically configured either as a series of plates or in a spirally wound electrode assembly. The two commonly used chemical systems are the lead acid system and the nickel cadmium system.
Although the lead acid battery system has been known and utilized for many decades, solutions to many of the practical difficulties associated with using such cells were not proposed until the mid-1970s. One of the difficulties seen with early lead acid cells was related to the problem of keeping the electrolyte acid contained within the cell. It was necessary to maintain an excess amount of acid (generally sulfuric acid) in the cell in order to allow for overcharging of the electrodes during the recharge process. Overcharging leads to the production of hydrogen and oxygen within the cell which traditionally was vented from the cell. Electrochemical cells having vent means and free acid generally had to be held upright in order to prevent the acid from leaking from the cell.
An additional problem with traditional lead acid cells was in maintaining the physical characteristics of the lead plates within the cell. Pure lead has some fluid flow and is also relatively flexible. In order to put some "back bone" in the lead plates, lead containing up to one percent of calcium was often used in cells. The calcium in the lead gives the plates some rigidity, but significantly reduces the efficiency of the discharge/recharge chemistry.
The breakthrough invention in lead acid cells is described in U.S. Pat. No. 3,862,861 of McClelland et al. The McClelland patent discloses the incorporation of several elements that combine to alleviate each of these problems associated with the traditional lead acid cell. The McClelland invention recognized the potential of utilizing the electrochemical recombination reaction between the oxygen and hydrogen formed during overcharging to maintain a balanced system. By capitalizing on the "oxygen cycle", a lead acid cell could be produced such that the electrolyte could be maintained in a "starved" condition. Rather than having an excess of electrolyte, the cell could be operated with a minimal amount of electrolyte present in the system. In order to maintain a starved condition, it is necessary to have sufficient absorbant material or pores within the cell to contain the electrolyte.
By using relatively absorptive separator material, McClelland was able to accomplish two distinct functions. The absorptive separator allowed the flow of gases and electrolyte between the positive and negative plates, thereby allowing the oxygen cycle to function. The absorptive separator also acts as a wick to hold the electrolyte within the cell without the necessity of having free electrolyte in the system.
McClelland also discloses a configuration of the plates and separator so that the elements are held tightly together. Fluid flow of the lead is thus prohibited. It was then possible to use considerably purer lead grids that are electrochemically more efficient than the calcium containing lead plates previously used. Venting means are included in the McClelland device as a safety release device in case, through some malfunction, gases generated during recharging were not reconverted to water. However, since there is little or no non-absorbed electrolyte in the cell, there is almost no danger of acid leaking from the cell.
Prior to the development of the McClelland device, U.S. Pat. Nos. 3,395,043 and 3,494,800 of Shoeld disclosed the use of relatively thin lead plates in an electrochemical cell. The cells described in the Shoeld patents, being prior in time to the McClelland patent, did not use absorptive, gas permeable separators. The cells disclosed did not, therefore, utilize the oxygen cycle, were not maintained in a starved or semi-starved condition, and probably contained free electrolyte in order to function properly. The Shoeld patents do not indicate that the batteries produced would have superior discharge or recharge characteristics. Based on the techniques and materials available at the time of the Shoeld disclosures, it is quite unlikely that the cell disclosed therein would have had any significant advantages over existing cells.
Since the McClelland patent, there have been several patents disclosing improvements to the fundamental cell disclosed therein. For example, U.S. Pat. Nos. 4,465,748 of Harris, 4,414,295 of Uba, 4,233,379 of Gross, 4,137,377 of McClelland and 4,216,280 of Kono each describe separators to be used in starved lead acid cells. U.S. Pat. Nos. 4,725,516 of Okada and 4,648,177 of Uba both identify cell parameters that lead to superior recharge/discharge characteristics in lead acid cells.
U.S. Pat. No. 4,769,299 of Nelson to a certain extent incorporates the inventions of Shoeld and McClelland. The Nelson patent describes the use of grid-like plates and absorptive gas permeable separators as described in McClelland with the extremely thin plates disclosed by Shoeld. The result is a lead acid cell with enhanced recharge/discharge properties.
The theoretical advantage of utilizing thin plates in electrochemical cells has been known for decades. The thinner the plates the less distance electrons have to travel within the plate during discharge, and, during recharge, the shorter distance of non-conductive material to be regenerated. To a certain extent, the thickness of plates utilized has been dictated by the available technology for the production and handling of thin lead films.
For much the same reasons that thin plates produce superior results, thin layers of reactive paste also lead to superior discharge/recharge characteristics. The Nelson patent discloses the use of both thin lead grids and thin layers of reactive paste. A basic shortcoming in the Nelson device, is that the paste residing within the grid openings can have a greatly increased distance to the lead plate material. For example, in the Nelson patent the openings in the lead plate grid are constructed so that the distance from the center of the grid to the grid strands is significantly greater than the thickness of the paste layer on the face of the plate. Since the performance characteristics of electrochemical cells is proportional to the thickness of the lead plates and the thickness of the paste layer, the use of grids greatly decreases the efficiency of the cells.
Typically, spirally rolled electrochemical cells are designed so that tabs are periodically incorporated into the plates--the tabs of one polarity going one way, the tabs of the opposite polarity going the other--in order to make connections from the plates to the cell terminals. This arrangement creates a problem in high rate discharge cells. The rapid discharge of substantial amounts of power generates a significant amount of heat along the tabs and terminals due to the relatively high resistance of the arrangement. U.S. Pat. No. 4,322,484 of Sugalski describes the use of an additional element within the cell to act as a heat sink.
Although there have been significant advances in the field of electrochemical cells, the theoretical possibilities for such systems have not been met.